Works are performed in an oil or gas well, among others, to stimulate or treat the well to increase production, to replace various equipment such as valves, to make measurements, to monitor the state of the well, or anything else being required.
Treatment of the well, for increasing the production rate or volume, is made after a cost/benefit evaluation. Even if the production from a well may be increased by several factors, the intervention costs may become to high or the work considered being to difficult and time consuming. For onshore or platform wells, having easy access into the Christmas tree and infrastructure in the form of lifting equipment etc., the costs of performing the well intervention will be less relatively to the benefit of the operations. The intervention of subsea wells is much more expensive. A vessel (drilling rig or the like) has to be used, involving large daily expenses and, in addition, time consuming transit to and from the field, and large costs as the work requires much more time. Because of this, the production volume from a platform or onshore well is also up to twice the volume of a subsea well with similar reservoir conditions. As mentioned above this is caused by the more easy access making a better programme for well maintenance practically possible and profitable.
Well intervention may be difficult, as existing barriers have to be removed before entering the well. There are strict rules regarding which measures being required to prevent an uncontrolled blowout during such works. Thus, when well intervention shall be performed, a pressure barrier has be established in the form of a blowout preventer. This may vary from simple stop valves to large drilling BOPs. In addition, circulating fluids in the well may be needed, whereby possible pressure increase in the well may be controlled.